PSA processes have long been used for the separation of the components of air. More recently, there has been considerable interest in the intensification of separation processes. In cyclic processes such as PSA and TSA, reducing cycle time is the primary means of achieving more production from a given quantity of material. However, as cycle time is reduced, cyclic processes usually face the problem of decreasing working capacity per cycle for the component of interest, decreasing product recovery, and increasing pressure drop.
Recent developments in PSA processes involve using adsorbents with faster adsorption kinetics, such as relatively fast kinetically selective laminate adsorbent structures, to increase productivity. However, such improvements in productivity generally come at the expense of reduced selectivity, resulting in reduced product recovery. Other developments include the use of adsorbents with relatively slow adsorption kinetics to improve overall product recovery of the processes. However, the improvement in product recovery generally comes at the expense of reduced process productivity.
U.S. Pat. No. 7,645,324 discloses a rotary PSA process using a laminated adsorbent for kinetic separation of gases. U.S. Pat. No. 7,645,324 teaches that the use of the kinetically selective laminate can allow for increased productivity, but in order to avoid the masking of kinetic selectivity by macropore mass transfer resistance the macropore structure within the adsorbent layer should be as open as possible; i.e., the macropore void fraction should be relatively high. A problem in this respect, however, is that having a high void volume generally harms product recovery.
U.S. Pat. No. 9,895,646 discloses a multi-bed PSA process for producing a gas stream enriched in a compound X from a feed gas stream. U.S. Pat. No. 9,895,646 notes that introducing pressure equalization steps into a PSA process improves product recovery, but doing so is generally detrimental to the specific productivity of the process. It is noted that moving from 1 to 3 equalizations makes it possible to gain 2.5% regarding the efficiency, but to the detriment of an increase of 40% in the volume of adsorbent (due to more adsorbent beds being required). Thus, increasing the number of adsorbent beds allows for increased product recovery (as more pressure equalization steps can be performed), however that also leads to a decrease in the specific productivity of the process (standard volumetric flow rate of product divided by total amount of adsorbent in the system).
WO 2015/199227 discloses a multi-bed (3 or more beds) PSA process for separating methane from biogas. The process performs a pressure equalization process of transferring the gas in an adsorption tower in which the adsorption process has been finished and which is in the high pressure state into another adsorption tower that is in a lower pressure state so as to bring the inside of the adsorption tower into an intermediate pressure state, and a pressure equalization process of receiving, after finishing the decompression process, the gas from another adsorption tower that is in a higher pressure state so as to bring the inside of the adsorption tower into an intermediate pressure state. This is said to improve the efficiency of the energy required for pressure increase and pressure lowering in the adsorption towers, and also improve the recovery rate of a gas to be purified, while improving the purity of the gas to be purified. However, adding the pressure equalization step did not improve the specific productivity of the process. No kinetic information is provided for the adsorbents used, however the requirement of a long pressure transfer step (6 seconds) indicates that a slow kinetic adsorbent was used.
To summarize the above, it is known that adsorbents that have relatively fast adsorption rates can increase process productivity, but that this generally comes at a cost of lower product recovery due to higher void volumes and/or reduced levels of selectivity. Introducing more adsorbent beds and pressure equalization steps into a process that uses said adsorbents could, in theory, improve upon that low product recovery rate, however it is expected that doing so would come at the cost of cancelling out the improvements in productivity intended to be obtained by using the faster adsorbents in the first place. Alternatively, a slower, more selective kinetic adsorbent can be used to obtain high purity product with good product recovery, however that also comes at the cost of reducing the overall productivity of the process.
It is therefore evident from the prior art that there is a trade-off between product recovery and process productivity; process modifications that increase product recovery are generally detrimental to process productivity, and vice versa.
Accordingly, there remains a need for PSA processes that have high process productivity whilst maintaining high product recovery.